Zirconium corundums, which are produced by extremely rapid cooling of a melt, are distinguished by particularly advantageous abrasive properties.
Thus, German Patent Application DE-A 2 519 569 describes an abrasive material based on Al.sub.2 O.sub.3 and ZrO.sub.2, optionally with accompanying substances or additions consisting of chromium, iron, titanium, vanadium, magnesium and/or rare earths in the form of oxides, carbides, carbonitrides and nitrides for use in precision grinding or semi-precision grinding. The additions are not used to increase grinding performance, but are supposed to be kept as small as possible to avoid reductions in quality.
German Patent Application DE-A 2 160 705 describes an abrasive material produced by melting a mixture of Al.sub.2 O.sub.3 with an addition of 5 to 45% by weight ZrO.sub.2, preferably 25% by weight ZrO.sub.2 and 0 to 4% by weight TiO.sub.2.
German Patent Application DE-A 2 227 462 describes a zirconium corundum which is cooled extremely rapidly by pouring a melt of Al.sub.2 O.sub.3 and ZrO.sub.2 prepared under reducing conditions between metal plates and which is eminently suitable for use in abrasives by virtue of the crystallite structure thus stabilized and the content of tetragonal high-temperature modification of ZrO.sub.2.
The phase transition of the ZrO.sub.2 during the grinding process from the tetragonal high-temperature modification into the monoclinic form stable at room temperature causes the appearance of transition-induced microcracks through the resulting increase in volume so that the material has to be strengthened by energy dissipation.
German Patent Application DE-A 3 040 992 describes a zirconium corundum containing 27 to 35% by weight ZrO.sub.2 and--expressed as TiO.sub.2 --1 to 10% by weight oxycarbides, carbides or suboxides of titanium obtained by reduction of TiO.sub.2. The performance of this abrasive material is characterized as equal to or better than that of the material described in DE-A 2 227 642. However, the advantage of this abrasive material is seen above all in the reduction in manufacturing costs for the same abrasive performance which is achieved by reduction of the ZrO.sub.2 content.
The mechanism on which this improvement in the properties of the abrasive material by the titanium compounds is based remains largely unresolved. It is assumed in DE-A 3 040 992 that the grinding process is directly influenced by the titanium compounds which are said to participate actively in the grinding process.
Both DE-A 3 040 992 and DE-A 2 227 642 disclose abrasive materials which contain at least 25% by weight, based on the ZrO.sub.2 content, of tetragonal ZrO.sub.2 modification.
U.S. Pat. No. 5,143,522 describes an abrasive grain with a content of 20-50% by weight of ZrO.sub.2, more than 25% of which is in the tetragonal form, 1-10% by weight of reduced titanium oxide in the form of suboxides, carbides or oxycarbides, 0.03-0.5% by weight of carbon, less than 3% by weight of impurities and Al.sub.2 O.sub.2 as the main constituent. The structure of the grain is described as consisting of primary Al.sub.2 O.sub.3 crystals embedded in a eutectic alumina-zirconia matrix. The size of the primary crystals is between 5 and 50 microns.
Japanese patent 1 614 974 (Appl. No. Sho 58-102289) describes zirconium corundums containing titanium dioxide in which up to 100% of the ZrO.sub.2 is present in the tetragonal modification. It is apparent from this document that the Y.sub.2 O.sub.3 also added is solely responsible for the stabilization and enrichment of the tetragonal phase. Thus, the Examples in the specification only show an increase over the undoped standard in the case of the samples doped with Y.sub.2 O.sub.3.
The addition of Y.sub.2 O.sub.3 adds to the production costs of the abrasive material. The abrasive performance improved by the high percentage contents of tetragonal ZrO.sub.2 modification is also limited by the fact that Y.sub.2 O.sub.3 or Y.sub.2 O.sub.3 -containing compounds do not themselves have any properties beneficial to the grinding process.
The object of the present invention is to provide a zirconium corundum abrasive material which would show improved performance compared with hitherto known zirconium corundums without any increase in production costs by comparison with conventional zirconium corundums so that, where the abrasive material according to the invention is used, an even more favorable price-to-performance ratio could be achieved for a number of grinding operations than was possible in the prior art.